Change Username in windows

hello everybody!
I have installed windows XP in my laptop. I want to change my user name (loginname) of my XP. I followed the following steps.
right click on Mycomputer->manage -> users and groups -> users ->
and then right click on my user name (pratapch) and say rename. It is allowing to type new name, but it is not accepting the new name. I tried in another laptop and there it is working. Then I tried in another laptop, there it is not working. I am getting confused what to do to rename the login name. I have administrative rights and the account which i tried to change is also administrator account (i am doing this from same account, working in one laptop and not in one).
Any help would be greatly appreciated.
Thanks in advance..